The present invention relates to optical transmission networks, optical communication systems or optical transmission systems, various optical transmission devices which include the optical amplifier systems used in those systems, and methods of controlling the systems and devices. More particularly, the present invention relates to an optical amplifier unit controlling method, an optical amplifier system, and a system which uses the method and system.
It is necessary to suppress light surges to the utmost in general optical amplifier systems. The "light surge" referred to herein points out an optical signal with an extremely high gain which is outputted from an optical amplifier system when the optical signal input to the optical amplifier system increases momentarily. The light surge is generated on the following reasons. It is necessary to expand the power of pumping light and to increase the amplification degree of an optical amplifier unit to obtain a desired optical signal output when the inputted optical signal decreases. Thus, in that case, large amplified optical signal energy is accumulated potentially in the optical amplifier unit. In such a state, if the optical signal input increases, the optical signal receives the energy accumulated so far and is outputted with a very high gain from the amplifier. If a light surge is generated, destruction of a photodetector in the optical communication end and melting of an end face of an connector concerned would be invited but also human (sight) injury would be caused. Therefore, it is necessary to suppress the generation of the light surge to the utmost. Especially, when optical amplifier systems are arranged in a multi-stage connection, the situation would be further serious. The reason for this is as follows: a light surge generated once is amplified one after another in the respective subsequent optical amplifier systems. As a result, the optical parts which compose each of those optical amplifier systems might be fatally destroyed with the respective increasing surges.
Current examples of measures against optical surges are described in the paper "Discussion of Light Surge in Multistageous Connection of Optical Amplifiers" (Spring Meeting B-941, Institute of Electronics, Information and Communication Engineers of Japan, 1993). The composition of an experimental system in the example of the measures is shown in FIG. 41A. The optical output level of each of the multistage-connected optical amplifiers is shown is FIG. 41B. As shown in FIG. 41A, an optical signal with available risetime can be generated from an laser diode (LD) (an LD module of the DFB (Distribution Feed Back) type having a center wavelength of 1.55 .mu.m) as the optical signal source by driving the LD with a current. Optical signals from that LD are passed sequentially via the amplifiers AMP1-AMP5, which are erbium doped optical fiber amplifiers which are pumped by a 1.48 .mu.m wavelength pump laser) with optical attenuators (ATTs) arranged before the corresponding amplifiers, and provided as an optical signal output. The waveforms of the respective optical signals outputted from those optical amplifiers are observed by corresponding photodiodes (PDs) via ATTs. As will be seen from FIG. 41B, the surge is suppressed in proportion to an increase in the risetime of the optical signal from the LD. Especially, when the risetime is set at the order of several milliseconds, light surges are hardly generated.
The amplifier composition of JP-A-6-45682 is shown in FIG. 42. As shown in FIG. 42, the optical signal multiplexed by an optical multiplexer 52 and pumping light from a laser diode 53 pass forwardly through the optical isolator 54 and enter a doped fiber 55. Then, the pumping light and the rare earth elements doped in the waveguide area causes induced emissions, and the optical signal is amplified. The amplified optical signal and the pumping light which remains unconsumed enter an optical bandpass filter 56. In the bandpass filter 56, the pumping light and spontaneous emission light which will be elements of noise are removed. The amplified optical signal alone passes an optical bandpass filter 56. Thereafter, a part of the optical signal is separated by an optical splitter 57, and the separated signal part is received by a photodetector 58. A bias control circuit 59 compares a direct current voltage from the photodetector 58 with a reference voltage Vref1 and controls a bias current flowing in the laser diode 53 so that an error between the direct current voltage and the reference voltage may become zero. Reference numeral 60 denotes a 4-port optical circulator having ports 60A, 60B, 60C and 60D. The light supplied to the port 60A is outputted only from the port 60B, the light supplied to the port 60B is outputted only from the port 60C, the light supplied to the port 60C is outputted only from port 60D, and the light supplied to the port 60D is outputted only from the port 60A. The control light from the laser diode 61 is supplied to the port 60A. The port 60B is connected with a port 57B of the optical splitter 57, the port 60C is connected with an output optical transmission path (not shown), and the port 60D is made a dead end. The control light from the laser diode 61 is introduced into the doped fiber 55 by passing the optical circulator 60, optical splitter 57, and optical band pass filter 56 in this order. Simultaneously, a bias control circuit 62 controls a bias current flowing through the laser diode 61 to thereby control the power of the control light from the laser diode 61 so that the error between a direct current voltage from the photodetector 58 and a reference voltage Vref4 may be zero.
According to the prior art JP-A-45682, the wavelength of the control light is set in a wavelength band where induced emission occurs in the doped fiber 55, for example at substantially the wavelength of the optical signal. When the power of the input signal changes comparatively slowly, the photodetector 58 receives a part of the optical signal which has passed through the optical bandpass filter 56. A bias control circuit 59 controls the power of the pumping light from the laser diode 54. When the power of the input optical signal changes rapidly, a bias control circuit 62 controls the power of the control light supplied by the laser diode 61. As a result, even if the input signal changes rapidly, the power of the output signal is kept constant.
In addition, the composition of the prior art JP-A-8-18138 is shown in FIG. 43. As shown in FIG. 43, in this composition, Two optical amplifiers AMP1 and AMP2 are connected in cascade. The first optical amplifier AMP1 is provided with a first pump source 102 composed of a first EDF, an LD, etc., a first multiplexer 104, and a first isolator 106. An optical signal input is applied to the first optical amplifier AMP1 via an optical isolator ISO connected with one end of the input side optical fiber. The first EDF 100 is pumped by the first pump source 102 via the first multiplexer 104. The optical signal which has passed the first EDF 100 is inputted to the second optical amplifier AMP2 through the first optical isolator 106.
The second optical amplifier has a second EDF 108, delay fiber 110, second pumping source 112, second multiplexer 114, second optical isolator 116, third EDF 118, attenuator 120, third optical isolator 122, first splitting coupler 124, second splitting coupler 126 and photodetector 128. The first splitting coupler 124 splits the light from the first isolator 106 into two light portions at a predetermined ratio. The first split light portion enters the second EDF 108 through the delay fiber 110. The second split light portion enters the third EDF 118 through the attenuator 120. The second EDF 108 is connected with the splitting coupler 126. The third EDF 118 is connected with the second splitting coupler 126 through the third optical isolator 122. The second EDF 108 is excited by the second pump source 112 through the second multiplexer 114. The output light of the second multiplexer 114 is outputted to the output optical fiber through the second optical isolator 116.
The third EDF 118, attenuator 120, and the third isolator 122 compose an optical path which, when light of more than a predetermined optical strength, for instance, a light surge pulse, inputs to the first splitting coupler 124, functions to give to the second EDF 108 light which passes in a direction opposite to that in which the optical signal which passes the second EDF 108 via the splitting coupler 126 to thereby decrease the gain of the second EDF 108. Specifically, the second EDF 108 is caused to perform induced emission in a direction opposite to that in which the optical signal passes. In this case, the delay fiber 110 delays light from the first splitting coupler 124 so that the induced emission may occur before the light surge pulse enters the second EDF 108 through delay fiber 110.
In summary, generation of the light surge in the second optical amplifier AMP 2 is prevented by the light surge pulse generated in the first optical amplifier AMP1.
However, the above-mentioned document "Discussion of an Optical Surgre in the Optical Amplifier Multi-Stage Connection" only a little describes a method of suppressing a light surge by controlling a risetime of the optical signal input. Even by application of the method of suppressing a light surge to an actual optical communication system, a light surge due to a cause other than the rise in the optical signal input cannot be suppressed. That is, the application of such suppression is considerably limited. When physical vibrations and impact are applied to an optical fiber which is in the state of optical signal transmission even if the risetime of the optical signal input is controlled, a light surge might be easily invited due to a momentary change in the optical signal power which is due to the physical vibrations and impact.
In addition to the above-mentioned defects, the power of the pumping light from the pump source is required to be decreased or the pumping light is required to be stopped temporarily in the conventional optical amplifier in order to suppress a light surge. In that case, the light surge is not suppressed at a decreasing speed of the pumping light power to be suppressed. Improvement of the control responsiveness cannot be expected. The reason for this is that the degree of suppression of the light surge is dependent on the energy accumulated before the optical signal which is inputted to the optical amplifier unit rises, the rising speed of the optical signal and its optical power, the suppressing speed of the light surge is lower than the decreasing speed of the pumping power. Thus, the pumping light output from the pump source is put in the state of a temporary stop until the light surge is suppressed to a preset value. This implies that there is actually a blank time in which the surge cannot be effectively suppressed only with the pump source. It also implies that the light surge continues to be generated during the blank time.
In addition, in the conventional optical amplider, it is necessary to greatly change the driving current to the pumping source to stabilize the optical signal output from the optical amplifier against a momentary change in the optical signal input to the optical amplifier. When the driving current changes greatly, the light surge would be a factor to lack the stability of the optical signal output and to deteriorate the S/N ratio of the entire optical amplifier, as a result of the oscillation wavelength in the pump source changing.
The prior art JP-A-6-45682 refers to the optical output control speed by the control light, but not to its power. Actually, consumption control of the energy accumulated excessively in the doped fiber rather than the control speed and the specified measures for the consumption control of the energy are required to suppress the light surge surely , but this prior art does not refer to that point.
Moreover, it is difficult to carry out the consumption control of the energy accumulated excessively in the doped fiber in the control light of this prior art. The reason for this is that the wavelength region of the LD of substantially the same wavelength as the optical signal used as the control light is approximately 0.1 nm or less in the optical fiber transmission, the control light is caused to pass a bandpass filter which filters out wavelength components other than the wavelength of the LD to enter the doped fiber, and independent pumping light is required to be prepared. Any of them cannot supply energy enough for suppression of the light surge.
It is natural to decrease the loss of the optical signal so as not to impair the original function of the optical amplifier. In this prior art, the loss of the power of the control light is further required to be decreased. Therefore, it is necessary to provide expensive optical parts such as optical circulators which multiplex and split both the output optical signal and the control light at a low loss, as shown by the prior art.
In addition, it is necessary to have a separate laser diode used for the control light and a super-high output LD, which would cause a new problem of development.
Moreover, since the noise figure and amplification gain which are the characteristic elements of the optical amplifier are greatly deteriorated by introduction of the control light, the introduction of the control light adversely affects the inherent characteristics of the optical amplifier. The prior art never refers to a method of avoiding this problem.
In addition, the prior art has a composition in which the photodetector 58 feeds back the monitor output to control the pumping light and the control light and detects the optical outputs from the doped fiber 55 and the bandpass filter 56 to suppress the light surge. Therefore, when the input signal power, for example, decreases, excessive energy has already been accumulated in doped fiber 55. Therefore, this would be a factor of a light surge, this phenominum cannot be detected by the photodetector 58 alone. Moreover, with the detection of the photodetector 58, the light surge which has already been generated cannot be suppressed. It is difficult for the entire optical amplifier to suppress a light surge securely at high speeds only by monitoring the output of the amplifier.
Moreover, although a change in the optical output is referred to, a method of coping with a changing light input itself which is a fundamental cause of the light surge is not found regrettably.
In addition, though a method of controlling the optical output of the optical amplifier is mentioned, control over the gain of the optical signal is not described.
Moreover, in the prior art JP-A-8-18138, if the light surge generated by the first optical amplifier does not exceeded a fixed level, the advantageous effect of the invention is not produced. That is, the first optical amplifier necessarily generates a light surge functionally. Therefore, the light surge generated by the first optical amplifier is input directly to the optical multiplexer and optical isolator provided after the first EDF. Therefore, there is a danger that the generated light surge may adversely affect, for example, impair these devices. As described also in the prior art, a method of controlling the first pump source such that in the actual system the optical output of the first optical amplifier AMP1 is split and the split output portion is monitored and maintained at a constant value is generally used. At this time, it can be easily imagined that there is a danger that the photodetector for monitoring purposes, etc., may be destroyed by the light surge generated by the first optical amplifier.
The use of a splitting coupler having a high splitting ratio to minimize the optical loss of the main optical signal cannot be avoided (In the prior art, two 1:10 optical couplers are used). As a result, not only the light surge which reaches the third EDF but also light introduced into the second EDF becomes slight. In addition, because the optical power is greatly attenuated by the third EDF and optical attenuator in the prior art, the optical power which can be actually introduced into the second EDF becomes slight. The prior art describes that the split light surge is rendered to have about -10 to -20 dBm. It is necessary to supply optical power of -10 dBm or more to suppress the light surge at a very safe level, as will be explained in detail as follows. An enough light surge suppression effect is not achieved by the prior art method.
On the other hand, if the amount of attenuation is decreased to improve the light surge suppression effect in the prior art, there is a possibility that a ring oscillation will normally occur through a loop including the first EDF, third EDF and the delay fiber. This is undesirable to ensure the reliability of the real system. After all, the effect of the optical purge suppression in this prior art is considerably limited.
In addition, we verified and confirmed that light having wavelengths (spontaneous emission components) other than the wavelength of an optical signal which is a main component of the light surge fills a large role of increasing the surge suppression effect. However, in the prior art, the light components other than the optical signal wavelength have low power, and hence they are absorbed and their advantageous effects cannot be obtained.
In the embodiment, the composition of a ring laser is shown. It is known well in this composition that the maximum gain of the EDF certainly becomes constant because the oscillation threshold is determined. However, no measures are mentioned which prevent a possible light surge from occurring by referring to the difference in level between the input optical signals (for instance, the difference between minus infinity and -20 dB from which the input optical signals rise respectively) at a gain below the maximum gain.